Hydrous ethanol, which is also known as industrial alcohol, may range from 170.degree. to 195.degree. U.S. proof (85 to 97.5 vol.% alcohol) but is most often commercially produced as 190.degree. U.S. proof or 95 vol.% alcohol. This product has many uses in the chemical, pharmaceutical and beverage industries. For applications in which the water content of hydrous ethanol cannot be tolerated, it is necessary to produce so-called anhydrous ethanol which is usually 199.degree. U.S. proof or 99.5 vol.% alcohol. For motor fuel purposes, either straight hydrous ethanol or straight anhydrous ethanol can be used directly, but only anhydrous ethanol can be mixed with gasoline because of phase separation problems encountered when water is present in gasohol (a blend of 10% anhydrous ethanol and 90% gasoline).
Hydrous ethanol may be separated from a dilute alcohol or beer feedstock by a stripping-rectifying operation in a single tower. The production of anhydrous ethanol, however, requires additional and more costly processing of hydrous ethanol. The most commonly used method comprises azeotropic distillation in a dehydrating tower using an entraining agent such as benzene so that the water in the hydrous ethanol is removed overhead as a ternary azeotrope and anhydrous ethanol is withdrawn as the bottom product. The azeotropic overhead is condensed and separated in a decanter from which an upper benzene-rich layer is returned to the dehydrating tower and a lower water-rich layer is fed to a stripping tower for recovery of benzene which is returned to the system.
Although hydrous ethanol is simpler and less costly to produce than anhydrous ethanol, there is still a pressing need to develop energy-efficient improvements in the distillation of the dilute alcohol feedstock, particularly where the hydrous ethanol is to be used as motor fuel. In the production of anhydrous ethanol by the usual azeotropic distillation, as described above, multiple distillation towers are required so that the opportunity is afforded to operate the first tower or beer still at a higher pressure than the other towers so that the overhead vapors from the first tower can be used as the heat supply for either or both of the other towers. This concept is disclosed in the Ricard et al U.S. Pat. Nos. 1,822,454 and 1,860,554.
In order to utilize the concept of heat re-use in the production of hydrous ethanol, it becomes necessary to split the stripping-rectifying step into two sections so that one tower can be operated at a higher pressure than the other while supplying dilute alcohol feedstock to the two towers in parallel.